Adding rebates to concrete slab

[i_am_fubar]

I'm going to pour a 3x6m slab soon. It'll be rebated for blockwork, 2 walls with 100mm and 2 walls with 215mm blocks.

Using 3x2 timber looks like it may work. At least for the 100mm rebate frame:

When it gets to the 215mm rebate, things look a bit more ropey:

I'm worried that either 1) the upper section of the form will start to float on the concrete and distort my level or 2) the concrete will flow under the upper section and run out of the form all together.

Any tips/tricks or bits of knowledge I'm missing here?

Cheers,
Fubar.

 

[Mrs Ten Names]

Use similar crosspieces to hold the upper length in place.
You could make some of those crosspieces reach to the lower rebated part, and fix them to that lower shuttering, with upstands,, to tie it all together.

 

[mrrusty]

Have a look at mine that worked well. I used roadpins to brace the shuttering. Don't underestimate the lateral force. My barrowmix concrete people mixed it stiffer first round the edge to stop it flowing under. They also trowelled it off and tamped it smooth. Great service! https://community.screwfix.com/threads/garage-work-in-progress.222282/

I also used release oil

 

[Ivor Windybottom]

I've used cooking oil before. Dirt cheap and non-toxic. Pour some into a tin then brush it on. Just don't pour the leftovers back into the bottle!

I can't make sense of the drawing though. Lines in space!

Wood will float out of concrete unless you stop it. Don't leave wood inside the concrete, it all needs to be removable.

 

[i_am_fubar]

Have a look at mine that worked well. I used roadpins to brace the shuttering. Don't underestimate the lateral force. My barrowmix concrete people mixed it stiffer first round the edge to stop it flowing under. They also trowelled it off and tamped it smooth. Great service! https://community.screwfix.com/threads/garage-work-in-progress.222282/

I also used release oil

This is a great pour, exactly what I'm after. I'm assuming the laterals will be obscene and plan to brace the ever living bejeebies out of it.

I take it the DPM is just to stop the slab staying wet.

And is there any under slab insulation?

 

[mrrusty]

My edge boards were 12 x 2's and the rebate edge was 3x2's as you see well braced with road pins and with the braces also screwed in to the edge boards. I found the road pins worked really well - you can wack them down deep through a hole in the braces. The DPM just to isolate the slab from the MOT underneath. No insulation as it's just a garage and there was no point because it's unheated.

The 12x2's because raised veg beds afterwards....